Traditionally, methods for producing micro devices have relied heavily on materials such as single crystal silicon and related processes such as plasma etching used in connection with integrated circuit fabrication. However, due to the mechanical nature of some micro devices, such as microelectromechanical devices or “MEMS” having both mechanical and electrical features formed on a single substrate as well as micromechanical devices in general, the performance of such devices may be limited by the intrinsic properties of these traditional integrated circuit based silicon substrate materials. Accordingly, alternative material systems such as metals have been considered by the present inventors as potential candidates for bulk micromechanical and MEMS devices because the relative ductility and other properties of metal substrates such as titanium can reduce the risk of failure associated with brittle silicon substrates and harsh environments including biological systems.
Earlier developments by the present inventors provided cyclic metal anisotropic reactive ion etching with oxidation methods, referred to as “MARIO” processes, for the production of bulk titanium MEMS and other devices that require higher fracture toughness and/or resistance to harsh environments than can be provided by traditional silicon based substrate materials. The MARIO processes are discussed in detail in co-pending U.S. Utility patent application Ser. No. 10/823,559, filed on Apr. 14, 2004, by Noel C. MacDonald and Marco F. Aimi, entitled METAL MEMS DEVICES AND METHODS OF MAKING SAME, now U.S. Utility Patent Application Publication Number 2004/0207074A1, published on Oct. 21, 2004, which application claims the benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/463,052, filed on Apr. 16, 2003, both of which applications are incorporated herein by reference. In addition to their relative fracture toughness and resistance to harsh environments, titanium based micro devices and MEMS have excellent biocompatibility due to the biocompatibility of titanium itself and show promise for use in vivo applications.
Outside of the earlier work of the present inventors, the majority of prior art research on titanium dry etching (i.e. plasma-based etching) practiced by others of skill in the art has been performed on thin films deposited on conventional semiconductor substrates (e.g. silicon, glass, etc), in which the primary functionality of the thin film was electrical rather than mechanical in nature. In general, these alternative prior art processes rely upon known fluorine- and/or chlorine-based chemistries to etch titanium thin films. Gases known in the art to be suitable for thin film titanium etching utilizing such prior art processes include: CCl4/O2 with additions of fluorine containing gases, CCl4/CCl2F2 with admixtures of O2, Cl2/BCl3; Cl2/N2, CF4, CF4/O2, SiCl4, SiCl4/CF4, and CHF3, CF4/O2, and SF6.
Although it is known in the art that micromechanical structures dry etched into titanium thin films have been demonstrated, and that the etched titanium thin films so produced can be used in microelectronics, realization of high aspect ratio structures (i.e. structures with heights far greater than their width) with such techniques is significantly limited due to film thickness limitations imposed by the deposition processes (generally 10 micrometers). Furthermore, such techniques are also often hampered by the detrimental residual stresses that can arise in these deposited thin films, which serve to distort and deform the structures once they are released from the constraint of the substrate below. High aspect ratio structures are desired in micromechanical applications for a number of reasons, including: a) to provide stiffness in the out-of-wafer plane direction to enhance structural robustness and durability, and to enable fabrication of large suspended structures that would be difficult if not impossible to realize with low aspect ratio thin film structures; b) to provide greater vertical surface area for high force capacitive actuation and enhanced sensing in MEMS actuators and sensors; and c) to provide greater mass for enhanced sensitivity in acceleration sensors. Accordingly, thin film titanium micro devices produced through the prior art techniques are generally unable to provide the functionality required for many micromechanical applications. Therefore, many are less than desirable for actual use outside or research relative to their silicon counterparts.
It is also known in the art that wet chemical and electrochemical-based etching methods have been demonstrated for fabrication of titanium-based micromechanical structures. In these techniques structures are generally etched into bulk titanium metal substrates rather than thin deposited films, thus enabling fabrication of structures with greater structural height. However, the aspect ratios that can be achieved using these techniques are also limited, due to the isotropic nature of the etching processes. This isotropy, characterized by similar rates of etching in all directions, causes undercutting of the masking materials which therefore precludes the fabrication of thin, high aspect ratio structures. This undercutting also prevents direct transferal of the mask features into the substrate therefore constraining the types of features and geometries that can be produced. Finally, undercutting also constrains the structural complexity that can be achieved because neighboring features must be spaced far enough apart to ensure that the desired etch depth will be achieved before the lateral undercutting undermines the etched structures. Such undercutting is also common in dry etching of bulk titanium substrates, which therefore provided the impetus for the development of the cyclic etch/passivation MARIO processes described earlier.
There are additional drawbacks in these earlier titanium etching processes that have further reduced the ability of such known titanium microdevices and MEMS to become competitive alternatives to traditional silicon-based devices. For example, as successful as the MARIO processes are at producing high aspect ratio titanium microdevices, they do so rather slowly. This is because of the relatively low etch rates provided by the MARIO processes resulting from their reliance upon cyclic, alternating protective oxidation steps sandwiched between reactive etching steps, in order to prevent isotropic lateral undercutting. In addition, there are rate limiting aspects inherent in the parallel plate, capacitively coupled plasma systems used in the MARIO processes.
Accordingly, there is a need in the art for improved bulk titanium etching and deep etching processes that will effectively produce high etching rates in titanium substrates of varying thickness for the fabrication of highly functional, robust, reliable, and even biocompatible, titanium-based devices composed of high aspect ratio micro-structural features with vertical sidewalls and smooth surfaces.